This invention relates to a convergence correcting device used in a cathode ray tube having coplanar in-line electron guns for producing multiple electron beams. The invention particularly relates to a convergence correcting device capable of coma correction.
The convergence correcting device for a cathode ray tube having in-line electron guns operates in such a way that an external magnetic field is applied to pole pieces provided for the electron guns and the electron beams between the pole pieces are rendered a deflection force through the adjustment of the direction and magnitude of the magnetic field.
FIG. 1 shows a cathode ray tube having in-line electron guns seen from above. The cathode ray tube 1 consists of a set of in-line electron guns 2, a neck section 3 surrounding the electron guns 2, convergence yokes 4, convergence coils 5, pole pieces 6 made of magnetic material, and deflection coil 7 for deflecting the electron beams in the horizontal and vertical directions. The arrows 8 indicate a leakage flux derived from the deflection coil 7. Electron beams emitted from the electron guns 2 have their trajectories corrected by a convergence correcting device made up of the yokes 4, coils 5 and pole pieces 6 so that they converge to a point on a shadow mask (not shown) behind of the screen.
FIG. 2 is a cross-sectional view of the neck section 3 taken along the line II--II of FIG. 1. With a current supplied to the convergence coils 5, the yokes 4 produce a magnetic field, which passes through the pole pieces 6 arranged at the neck section 3 of the tube 1 in close vicinity to the yokes 4, and magnetic paths shown by the dashed lines are formed. Accordingly, perpendicular magnetic fields are produced between the upper and lower pole pieces 6 as shown. The electron beams B and R in the magnetic fields are rendered a horizontal deflection force in proportion to the magnitude of the magnetic fields. The deflection force is used to move the vertical rasters 1R and 1B to the respective directions indicated by the arrows, and all three rasters 1R, 1B and 1G are converged on a line as shown in FIG. 3.
What should be noted here is that part of the leakage flux 8 from the deflection coil 7 passes by a location near the pole pieces 6. Namely, the magnetic field 8 passes through the pole pieces 6 having a small magnetic reluctance, forming a magnetic field shown by the arrows of solid lines in FIG. 2. This magnetic field, shown by the solid lines, virtually acts on only the central electron beam G, and consequently the horizontal raster 2G produced by the central beam G separates from the horizontal rasters 2B and 2R produced by the outermost beams B and R at the top and bottom of the screen, as shown in FIG. 4. The result is the creation of coma in the vertical direction.
The convergence correcting device arranged as described above is disclosed, for example, in Japanese patent examined publication No. 47-9939 filed on Mar. 13, 1968 in Japan. Another convergence correcting device is described in Japanese patent examined publication No. 50-27966 filed on Aug. 23, 1967 in Japan by General Electric Company under priority right based on U.S. patent application Ser. No. 574,411 filed on Aug. 23, 1966. This publication discloses the arrangement of a convergence yoke and pole pieces in combination for correcting mis-convergence of vertical rasters and another combination of a convergence yoke and pole pieces for correcting mis-convergence of horizontal rasters. The above invention has problems of the need of a large area for mounting two sets of convergence yoke and pole pieces, a complex structure and an increased number of component parts.
There has been proposed the provision of a shielding plate between the pole pieces 6 and the deflection yoke 7 with the intention of shielding the leakage flux from the deflection coil shown in FIG. 1. In this arrangement, however, the shielding plate provided at the rear end of the deflection yoke acts to cancel part of a leakage flux produced by the main winding of the horizontal and vertical deflection coils, which negates the contribution of the horizontal and vertical deflection magnetic fields in the rear of the deflection yoke to the deflection of the electron beams, resulting in a degraded horizontal and vertical deflection sensitivity and also in an increased pincushion distortion due to the shift of the deflection start point into the screen.